This application relates to a heated pressure fusing apparatus used in xerographic copying machines and in particular to an improved pressure roll used in conjunction with a fuser roll for providing a nip through which copy sheets are moved so that toner images contact the fuser roll.
Generally in xerography, a xerographic surface comprising a layer of photoconductive insulating material affixed to a conductive backing is used to support electrostatic images. In the usual method of carrying out the process the xerographic surface is electrostatically charged uniformly across its surface and then exposed to a light pattern of the image being reproduced to thereby discharge the charge in the areas where the light strikes the layer. The undischarged areas of the layer thus form an electrostatic charge pattern in conformity with the configuration of the original light pattern. The latent electrostatic image is developed by contacting it with a finely divided electrostatically attractable powder (toner). The powder is held in image areas by the electrostatic charges on the layer. It is then transferred to a sheet of paper or other suitable surface and affixed thereto to form a permanent print.
There are various ways of fusing or affixing the toner particles to the support member, one of which is by the employment of heat. In order to affix or fuse electroscopic toner materials permanently onto a support member by heat, it is necessary to elevate the temperature of the toner material to a point at which the constituents of the toner material coalesce and become tacky. This action causes the toner to adhere to the support member. In both xerographic as well as the electrographic recording arts, the use of thermal energy for fixing toner images onto a support member is old and well known.
One approach to thermal fusing of electroscopic toner images onto a support has been to pass the support with the toner images thereon between a pair of opposed roller members, at least one of which is either externally or internally heated. During operation of a fusing system of this type, the support member to which the toner images are electrostatically adhered, is moved through the nip formed between the rolls with the toner images contacting the fuser roll to effect heating of the toner images within the nip. In order to enhance fusing of the toner images in the foregoing manner, the pressure or backup roll of the fuser roll pair is usually constructed so that the fuser roll creates a depression in the pressure or backup roll as the result of a biasing force which forces the rolls into engagement. To this end the pressure or backup roll comprises a rigid core having a relatively thick resilient layer affixed thereto and an outer layer or sleeve of abhesive material. The abhesive material exhibits a low affinity for tackified toner. The aforementioned depression is continually formed as different portions of the pressure or backup roll move into and out of engagement resulting in a large number of flexures of the relatively thick resilient layer and the outer layer. The useful life of such pressure or backup rolls depends to a large degree on the ability of the materials forming the layers to withstand the strain of continued flexing.
Typical devices for fixing the toner particles to the sheet by a heated pressure fusing roll apparatus in which the copy sheet passes through the nip of a coated heated fuser roll and a pressure or backup roll are described for example in U.S. Pat. Nos. 3,256,002; 3,268,351; 3,841,827; and 3,912,901. In U.S. Pat. No. 3,912,901, Strella et al describe and claim pressure rolls comprising a rigid core; a layer of resilient material adhered to the rigid core; and an outer layer over the resilient layer, the outer layer comprising a copolymer of perfluoroalkyl perfluorovinyl ether with tetrafluoroethylene. Strella et al disclose that the elastomeric resilient material is a heat-resistant, organosiloxane polymer commonly known as silicone rubber. Silicone rubber is generally considered adequate for this purpose, and pressure rolls prepared with silicone rubber as the resilient layer generally perform as pressure rolls for a substantial number of hours, especially when coated with the sleeve material of a copolymer of perfluoroalkyl perfluorovinyl ether with tetrafluoroethylene as described by Strella et al. However, the pressure rolls having a silicone rubber resilient layer must be end capped so that the silicone rubber will not be impacted by silicone oil or fluids which are normally applied as offset preventing liquids or fluids to the outer surface of the fuser roll. When silicone rubber is exposed to silicone oil, the silicone rubber swells, and the integrity of the rubber deteriorates thereby decreasing its effectiveness under the pressures and temperatures normally encountered in the pressure fusing systems. Silicone oil applied to the fuser roll eventually carries over to the pressure roll causing the foregoing disadvantages unless the pressure rolls are end capped to prevent exposure of the silicone rubber resilient layer to silicone oil. This precaution results in added expense in the preparation of pressure rolls.
Furthermore, silicone rubbers encased in a sleeve inherently soften substantially with use, especially under the pressure and high temperatures required for the pressure fixing or fusing of toners. This softening reduces the useful and effective life of pressure rolls having a silicone rubber resilient layer.
Heretofore, copolymers of perfluoroalkyl perfluorovinyl ether and tetrafluoroethylene were preferred as an outer sleeve material to cover the silicone rubber resilient material adhered to the core of a pressure roll to provide adequate pressure roll life especially in high speed copiers. This type of pressure roll is described by Strella et al in U.S. Pat. No. 3,912,901. Strella et al indicate that for certain machines, fluorinated ethylene propylene (FEP) is appropriate as an outer layer for pressure rolls in certain machines, however, Strella et al indicate that as operating parameters of copiers, such as copier speed, increase significantly, the flex fatigue life of FEP sleeves is not satisfactory and FEP cannot be used as the outer sleeve over silicone rubber in pressure rolls.
Another disadvantage of the pressure rolls made with a silicone rubber resilient layer and an outer sleeve or layer of a copolymer of perfluoroalkyl perfluorovinyl ether and tetrafluoroethylene is the relatively high cost resulting from the expensive materials.